We have recently received two more SSD drives from Corsair based on the second generation SandForce controller. Let’s check out their performance and also investigate how SSD speed scales depending on their storage capacity.

Testbed and Methods

The testbed we are going to use for this test session is based on a mainboard with Intel H67 chipset which offers two SATA 6 Gbit/s ports. It is to these ports that we connect our SSDs. Here is the full configuration:

Performance Secret - Interleaving

Corsair’s 240GB SSDs rely on 4-way interleaving in delivering higher performance compared to their lower-capacity cousins. The point of interleaving is that there can be from one to eight NAND flash devices (chips) connected to a single channel of the SandForce controller. The latter can access those devices individually. So when one of the flash memory chips is busy doing some operation (which can take up to a few cycles to complete), the respective controller channel does not stay idle but is used to access another memory chip. This full utilization of all controller channels leads to higher performance. The technique is most effective for writing, masking the latencies associated with storing data into flash memory.

Thus, the more NAND devices per channel the controller has, the higher performance it can deliver. Considering that the typical capacity of a single 25nm flash memory die is 64 gigabits, the level of interleaving increases along with the capacity of the whole SSD. For example, 240GB SSDs with modern 25nm flash contain 32 NAND devices, which means 4 such devices per controller channel. That’s why the Force GT and Force 3 series products we are going to test today make use of 4-way interleaving.

Interleaving does not multiply performance, though. Why? Well, first of all, it is just not possible at all in some situations, for example at reading. And even when erasing and writing data, the controller has to access flash chips in each channel one by one rather than in parallel, so some delays are unavoidable.

You can’t estimate the effect of interleaving basing on theoretical assumptions only, so we carried out a practical test to compare 120 and 240GB versions of Corsair’s Force 3 and Force GT SSDs. We used CrystalDiskMark 3.0.1 as it allows benchmarking SSDs with incompressible random data as well as with compressible predefined data. Therefore, there are two numbers in the diagrams: the maximum and minimum speed of each SSD. The real performance is going to be in between these two numbers and depend on how well the SF-2281/SF-2282 controller can compress data.

The 4-way interleaving implemented in the 240GB SSDs really makes them faster, especially at sequential writing. The performance boost is 70 to 90% depending on flash memory type (asynchronous or synchronous), but you can only achieve it with incompressible data. When it comes to compressible data, the SF-2281/SF-2282 controller can successfully mask the difference between fast and slow varieties of flash memory.

Interleaving also improves the speed of random-address access with a long request queue. Random reading and writing are both accelerated, and the random read speed even improves with incompressible data. The performance gain varies from 55 to 85%, being more noticeable with the faster Force GT series. As for ordinary random-address reading and writing, the larger-capacity SSDs with higher level of interleaving do not have any advantage then.

So, it turns out that Corsair SSDs from the same series have absolutely different speed specs if they have different capacities. The official specs for the 120GB and 240GB models are very close, yet the practical difference is huge from a consumer's standpoint. The larger-capacity SSDs are much faster even though they use the same components as their lower-capacity cousins from the same series. That's why you may want to check out not only the controller and flash memory type but also the memory architecture of the SSD you are going to buy. Besides the number of controller channels, interleaving has a huge effect on performance as you've just seen.